1. Field of the Invention
The present invention relates to an optical pickup device for reproducing and recording information from and on an optical recording medium and an optical recording and reproducing apparatus including the optical pickup device.
2. Description of the Related Art
Optical recording mediums such as an optical disc and an optical memory card are widely used as storage mediums for storing therein video information, audio information or programs for use with information equipment and so on.
As these optical recording mediums are progressively increasing their recording densities and storage capacities, an optical pickup device is increasingly reducing a diameter of a beam spot focused on an optical recording medium through an objective lens either by reducing a wavelength of laser light from a light source, e.g. a semiconductor laser or by increasing an NA (numerical aperture) of an objective lens.
For example, while a CD (compact disc), which became commercially available on the market in the relatively early stage, sets a wavelength of light emitted from a light source to 780 nm, a DVD (digital versatile disc), which became commercially available on the market in the later stage, sets a wavelength of light emitted from a light source to 650 nm or 635 nm.
In recent years, as a demand for realizing higher recording density and larger storage capacity is increasing, a wavelength of light emitted from a light source tends to become shorter increasingly.
Concurrently therewith, in order to increase an information recording capacity of an optical recording medium, a wavelength of light emitted from a light source should be reduced much more, and it is proposed to construct an optical pickup device by a blue semiconductor laser (LD) having a wavelength ranging of from 400 nm to 415 nm.
This previously-proposed optical pickup device using this blue semiconductor laser as a light source includes a collimator lens located between a light source and an objective lens for focusing light on an optical recording medium (optical disc, etc.), to collimate light emitted from the blue semiconductor laser of the light to provide collimated light.
A distance between the lens and the laser light emission point is adjusted in such a manner that the laser light emission point may become identical with the position of the focal point of the collimator lens.
However, since it is customary for the optical pickup device to make a holding member (base member) for holding thereon optical assemblies by aluminum, magnesium and the like and these materials are expanded and contracted due to ambient temperature, it is mechanically unavoidable that the position of the focal point of the collimator lens and the light emission point of the laser will not become identical with each other due to change of temperature.
Unless the light emission point of the laser becomes identical with the position of the focal point of the collimator lens as described above, light emitted from the collimator lens is no longer collimated light but becomes diverging light or converging light. If light in the above diverged or converged state is focused by the objective lens, then optical aberration occurs so that information cannot be accurately read out from the optical recording medium any longer.
Various technologies have been so far devised to correct a space between the collimator lens and the light emission point of the laser so that this space will not change optically, i.e., the position of the focal point of the lens may follow movement of the light emission point of the laser even when temperature fluctuates.
As a method for correcting temperature fluctuation, it is proposed to select material of lens and the number of lens by making good use of optical properties and mechanical properties of mainly material of lens in a lens design.
Variables that should be taken into consideration in a lens design may be (1) expansion and contraction of an optical assembly attachment base (holding member) due to fluctuation of temperature; (2) fluctuation of a refractive index of a lens due to fluctuation of temperature; (3) fluctuation of a refractive index of a lens due to fluctuation of a wavelength of light from a semiconductor laser in accordance with change of temperature; and (4) expansion and contraction of a lens itself due to change of temperature.
Since in most cases it is sufficient to set material of collimator lens and lens configuration such as the number of lens so as to cope with fluctuation of temperature in the wavelength region of a red semiconductor laser (wavelength 630 nm to 660 nm) for use with a DVD (digital versatile disc), for example, in most cases, freedom in an optical design was large and there were many solutions that can satisfy the conditions.
On the other hand, when a blue semiconductor laser is used as a light source, freedom in an optical design is limited extremely.
The reason for this is that when the blue semiconductor laser is used as the light source, chromatic aberration increases considerably as compared with the case of the red semiconductor laser so that not only fluctuation of temperature should be corrected but also chromatic aberration should be corrected.
Since chromatic aberration occurs due to wavelength dependence of refractive index of lens and the semiconductor laser has properties to fluctuate its wavelength in accordance with intensity of emitted light, the refractive index of the lens changes in response to the fluctuation of wavelength, and hence the focal length of lens changes unavoidably.
For example, upon recording, intensity of recording laser output increases several tens of times as high as the intensity of reproducing laser output, and hence the oscillation wavelength also changes from the oscillation wavelength required upon playback.
Accordingly, both in the recording mode and the reproducing mode, fluctuation of temperature and chromatic aberration should be corrected in such a manner that the position of the focal point of the collimator lens may become identical with the light emission point of the laser.
Therefore, an optical pickup device using a blue semiconductor laser light source needs an arrangement by which both of fluctuation of temperature and chromatic aberration can be corrected.
In order to suppress chromatic aberration, it has been proposed to use a special glass material of properties in which a refractive index less fluctuates relative to fluctuation of wavelength (see cited patent reference 1) and to construct a collimator lens by a large number of lens groups composed of more than two kinds of glass materials with different optical properties.
[Cited Patent Reference 1]
Official gazette of Japanese laid-open patent application No. 2001-243650
However, if a special glass material is used to form a collimator lens and a collimator lens is consists of lens groups comprising a large number of elements, then a cost of assemblies of the collimator lens increases and the optical pickup device also becomes costly.
Considering the optical pickup device from a money standpoint, it is desired that a lens configuration of the collimator lens should be a simple configuration such as a cemented lens comprising one element or two elements, one group at most.
When the blue semiconductor laser is used as the light source, both of the fluctuation of temperature and the chromatic aberration should be corrected. Further, considering a cost of a lens, the kind of lens materials that can be applied to the collimator lens is limited considerably, and hence lens design becomes very difficult.
Moreover, since a depth of focus of the collimator lens is proportional to a wavelength λ, if the blue semiconductor laser is used as the light source, then a depth of focus decreases as compared with the case in which the red semiconductor laser is used as the light source. As a consequence, a tolerance of errors caused in the position of the focal point of the collimator lens relative to the light emission point of the laser when the temperature fluctuates and the wavelength fluctuates (chromatic aberration occurs) becomes extremely limited.
For this reason, the optical pickup device needs an optical system capable of highly-accurately compensating for the focal point position displaced when temperature fluctuates and the wavelength fluctuates.